Mouse sperm injected into a modified, inactive embryo can generate healthy offspring, shows a paper in Nature Communications. The results suggest that sperm maturation, previously thought only to arise in the egg, can occur in absence of an egg, spurring researchers to look into how this alternative developmental process may arise.

Fertilization integrates multiple processes to transform a sperm and an egg into an embryo. During fertilization various chromosomal and DNA changes occur (a process known as reprogramming) that allow sperm to mature in order to divide and produce all the differentiated cells in an organism (this ability is called totipotency). However, the general view is that sperm can only be reprogrammed to totipotency within an egg.

Anthony Perry and colleagues inject sperm nuclei into chemically modified mouse embryos before their first division into two cells. The embryos were chemically treated so that they would contain just a single set of unpaired chromosomes, rather than the paired set usually created by the fusion of a sperm and an egg. The resultant embryos developed into healthy offspring (but only at up to 24% of control rates). Although the authors do not show how the sperm genome is reprogrammed, they see some chromosomal and DNA-based similarities between their modified embryos and control (uninjected) embryos, but also differences in cell processes suggesting a different route for reprogramming.

Although these results show that sperm maturation in the egg can be bypassed in certain circumstances, this is far from being applicable to human embryos. Firstly, survival in implanted embryos is low. Secondly, this study has been performed in mouse embryos and provides no evidence that this would work in human embryos.